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Compound statements contain (groups of) other statements; they affect or control
the execution of those other statements in some way. In general, compound
statements span multiple lines, although in simple incarnations a whole compound
statement may be contained in one line.

The if, while and for statements implement
traditional control flow constructs. try specifies exception
handlers and/or cleanup code for a group of statements, while the
with statement allows the execution of initialization and
finalization code around a block of code. Function and class definitions are
also syntactically compound statements.

A compound statement consists of one or more ‘clauses.’ A clause consists of a
header and a ‘suite.’ The clause headers of a particular compound statement are
all at the same indentation level. Each clause header begins with a uniquely
identifying keyword and ends with a colon. A suite is a group of statements
controlled by a clause. A suite can be one or more semicolon-separated simple
statements on the same line as the header, following the header’s colon, or it
can be one or more indented statements on subsequent lines. Only the latter
form of a suite can contain nested compound statements; the following is illegal,
mostly because it wouldn’t be clear to which if clause a following
else clause would belong:

iftest1:iftest2:print(x)

Also note that the semicolon binds tighter than the colon in this context, so
that in the following example, either all or none of the print() calls are
executed:

Note that statements always end in a NEWLINE possibly followed by a
DEDENT. Also note that optional continuation clauses always begin with a
keyword that cannot start a statement, thus there are no ambiguities (the
‘dangling else’ problem is solved in Python by requiring nested
if statements to be indented).

The formatting of the grammar rules in the following sections places each clause
on a separate line for clarity.

It selects exactly one of the suites by evaluating the expressions one by one
until one is found to be true (see section Boolean operations for the definition of
true and false); then that suite is executed (and no other part of the
if statement is executed or evaluated). If all expressions are
false, the suite of the else clause, if present, is executed.

This repeatedly tests the expression and, if it is true, executes the first
suite; if the expression is false (which may be the first time it is tested) the
suite of the else clause, if present, is executed and the loop
terminates.

A break statement executed in the first suite terminates the loop
without executing the else clause’s suite. A continue
statement executed in the first suite skips the rest of the suite and goes back
to testing the expression.

The expression list is evaluated once; it should yield an iterable object. An
iterator is created for the result of the expression_list. The suite is
then executed once for each item provided by the iterator, in the order returned
by the iterator. Each item in turn is assigned to the target list using the
standard rules for assignments (see Assignment statements), and then the suite is
executed. When the items are exhausted (which is immediately when the sequence
is empty or an iterator raises a StopIteration exception), the suite in
the else clause, if present, is executed, and the loop terminates.

A break statement executed in the first suite terminates the loop
without executing the else clause’s suite. A continue
statement executed in the first suite skips the rest of the suite and continues
with the next item, or with the else clause if there is no next
item.

The for-loop makes assignments to the variables(s) in the target list.
This overwrites all previous assignments to those variables including
those made in the suite of the for-loop:

foriinrange(10):print(i)i=5# this will not affect the for-loop# because i will be overwritten with the next# index in the range

Names in the target list are not deleted when the loop is finished, but if the
sequence is empty, they will not have been assigned to at all by the loop. Hint:
the built-in function range() returns an iterator of integers suitable to
emulate the effect of Pascal’s fori:=atobdo; e.g., list(range(3))
returns the list [0,1,2].

Note

There is a subtlety when the sequence is being modified by the loop (this can
only occur for mutable sequences, i.e. lists). An internal counter is used
to keep track of which item is used next, and this is incremented on each
iteration. When this counter has reached the length of the sequence the loop
terminates. This means that if the suite deletes the current (or a previous)
item from the sequence, the next item will be skipped (since it gets the
index of the current item which has already been treated). Likewise, if the
suite inserts an item in the sequence before the current item, the current
item will be treated again the next time through the loop. This can lead to
nasty bugs that can be avoided by making a temporary copy using a slice of
the whole sequence, e.g.,

The except clause(s) specify one or more exception handlers. When no
exception occurs in the try clause, no exception handler is executed.
When an exception occurs in the try suite, a search for an exception
handler is started. This search inspects the except clauses in turn until one
is found that matches the exception. An expression-less except clause, if
present, must be last; it matches any exception. For an except clause with an
expression, that expression is evaluated, and the clause matches the exception
if the resulting object is “compatible” with the exception. An object is
compatible with an exception if it is the class or a base class of the exception
object or a tuple containing an item compatible with the exception.

If no except clause matches the exception, the search for an exception handler
continues in the surrounding code and on the invocation stack. [1]

If the evaluation of an expression in the header of an except clause raises an
exception, the original search for a handler is canceled and a search starts for
the new exception in the surrounding code and on the call stack (it is treated
as if the entire try statement raised the exception).

When a matching except clause is found, the exception is assigned to the target
specified after the as keyword in that except clause, if present, and
the except clause’s suite is executed. All except clauses must have an
executable block. When the end of this block is reached, execution continues
normally after the entire try statement. (This means that if two nested
handlers exist for the same exception, and the exception occurs in the try
clause of the inner handler, the outer handler will not handle the exception.)

When an exception has been assigned using astarget, it is cleared at the
end of the except clause. This is as if

exceptEasN:foo

was translated to

exceptEasN:try:foofinally:delN

This means the exception must be assigned to a different name to be able to
refer to it after the except clause. Exceptions are cleared because with the
traceback attached to them, they form a reference cycle with the stack frame,
keeping all locals in that frame alive until the next garbage collection occurs.

Before an except clause’s suite is executed, details about the exception are
stored in the sys module and can be accessed via sys.exc_info().
sys.exc_info() returns a 3-tuple consisting of the exception class, the
exception instance and a traceback object (see section The standard type hierarchy) identifying
the point in the program where the exception occurred. sys.exc_info()
values are restored to their previous values (before the call) when returning
from a function that handled an exception.

The optional else clause is executed if and when control flows off
the end of the try clause. [2] Exceptions in the else
clause are not handled by the preceding except clauses.

If finally is present, it specifies a ‘cleanup’ handler. The
try clause is executed, including any except and
else clauses. If an exception occurs in any of the clauses and is
not handled, the exception is temporarily saved. The finally clause
is executed. If there is a saved exception it is re-raised at the end of the
finally clause. If the finally clause raises another
exception, the saved exception is set as the context of the new exception.
If the finally clause executes a return or break
statement, the saved exception is discarded:

>>> deff():... try:... 1/0... finally:... return42...>>> f()42

The exception information is not available to the program during execution of
the finally clause.

When a return, break or continue statement is
executed in the try suite of a try…finally
statement, the finally clause is also executed ‘on the way out.’ A
continue statement is illegal in the finally clause. (The
reason is a problem with the current implementation — this restriction may be
lifted in the future).

The return value of a function is determined by the last return
statement executed. Since the finally clause always executes, a
return statement executed in the finally clause will
always be the last one executed:

If a target was included in the with statement, the return value
from __enter__() is assigned to it.

Note

The with statement guarantees that if the __enter__()
method returns without an error, then __exit__() will always be
called. Thus, if an error occurs during the assignment to the target list,
it will be treated the same as an error occurring within the suite would
be. See step 6 below.

The suite is executed.

The context manager’s __exit__() method is invoked. If an exception
caused the suite to be exited, its type, value, and traceback are passed as
arguments to __exit__(). Otherwise, three None arguments are
supplied.

If the suite was exited due to an exception, and the return value from the
__exit__() method was false, the exception is reraised. If the return
value was true, the exception is suppressed, and execution continues with the
statement following the with statement.

If the suite was exited for any reason other than an exception, the return
value from __exit__() is ignored, and execution proceeds at the normal
location for the kind of exit that was taken.

With more than one item, the context managers are processed as if multiple
with statements were nested:

A function definition is an executable statement. Its execution binds the
function name in the current local namespace to a function object (a wrapper
around the executable code for the function). This function object contains a
reference to the current global namespace as the global namespace to be used
when the function is called.

The function definition does not execute the function body; this gets executed
only when the function is called. [3]

A function definition may be wrapped by one or more decorator expressions.
Decorator expressions are evaluated when the function is defined, in the scope
that contains the function definition. The result must be a callable, which is
invoked with the function object as the only argument. The returned value is
bound to the function name instead of the function object. Multiple decorators
are applied in nested fashion. For example, the following code

@f1(arg)@f2deffunc():pass

is equivalent to

deffunc():passfunc=f1(arg)(f2(func))

When one or more parameters have the form parameter=expression, the function is said to have “default parameter values.” For a
parameter with a default value, the corresponding argument may be
omitted from a call, in which
case the parameter’s default value is substituted. If a parameter has a default
value, all following parameters up until the “*” must also have a default
value — this is a syntactic restriction that is not expressed by the grammar.

Default parameter values are evaluated from left to right when the function
definition is executed. This means that the expression is evaluated once, when
the function is defined, and that the same “pre-computed” value is used for each
call. This is especially important to understand when a default parameter is a
mutable object, such as a list or a dictionary: if the function modifies the
object (e.g. by appending an item to a list), the default value is in effect
modified. This is generally not what was intended. A way around this is to use
None as the default, and explicitly test for it in the body of the function,
e.g.:

defwhats_on_the_telly(penguin=None):ifpenguinisNone:penguin=[]penguin.append("property of the zoo")returnpenguin

Function call semantics are described in more detail in section Calls. A
function call always assigns values to all parameters mentioned in the parameter
list, either from position arguments, from keyword arguments, or from default
values. If the form “*identifier” is present, it is initialized to a tuple
receiving any excess positional parameters, defaulting to the empty tuple. If
the form “**identifier” is present, it is initialized to a new dictionary
receiving any excess keyword arguments, defaulting to a new empty dictionary.
Parameters after “*” or “*identifier” are keyword-only parameters and
may only be passed used keyword arguments.

Parameters may have annotations of the form “:expression” following the
parameter name. Any parameter may have an annotation even those of the form
*identifier or **identifier. Functions may have “return” annotation of
the form “->expression” after the parameter list. These annotations can be
any valid Python expression and are evaluated when the function definition is
executed. Annotations may be evaluated in a different order than they appear in
the source code. The presence of annotations does not change the semantics of a
function. The annotation values are available as values of a dictionary keyed
by the parameters’ names in the __annotations__ attribute of the
function object.

It is also possible to create anonymous functions (functions not bound to a
name), for immediate use in expressions. This uses lambda expressions, described in
section Lambdas. Note that the lambda expression is merely a shorthand for a
simplified function definition; a function defined in a “def”
statement can be passed around or assigned to another name just like a function
defined by a lambda expression. The “def” form is actually more powerful
since it allows the execution of multiple statements and annotations.

Programmer’s note: Functions are first-class objects. A “def” statement
executed inside a function definition defines a local function that can be
returned or passed around. Free variables used in the nested function can
access the local variables of the function containing the def. See section
Naming and binding for details.

A class definition is an executable statement. The inheritance list usually
gives a list of base classes (see Customizing class creation for more advanced uses), so
each item in the list should evaluate to a class object which allows
subclassing. Classes without an inheritance list inherit, by default, from the
base class object; hence,

classFoo:pass

is equivalent to

classFoo(object):pass

The class’s suite is then executed in a new execution frame (see Naming and binding),
using a newly created local namespace and the original global namespace.
(Usually, the suite contains mostly function definitions.) When the class’s
suite finishes execution, its execution frame is discarded but its local
namespace is saved. [4] A class object is then created using the inheritance
list for the base classes and the saved local namespace for the attribute
dictionary. The class name is bound to this class object in the original local
namespace.

The evaluation rules for the decorator expressions are the same as for function
decorators. The result must be a class object, which is then bound to the class
name.

Programmer’s note: Variables defined in the class definition are class
attributes; they are shared by instances. Instance attributes can be set in a
method with self.name=value. Both class and instance attributes are
accessible through the notation “self.name”, and an instance attribute hides
a class attribute with the same name when accessed in this way. Class
attributes can be used as defaults for instance attributes, but using mutable
values there can lead to unexpected results. Descriptors
can be used to create instance variables with different implementation details.